1. Field of the Invention
The present invention is related to a method of and an apparatus for performing detail enhancement to improve sharpness of the image.
2. Description of the Prior Art
In detail enhancement, a so-called sharp signal S and a so-called unsharp masking signal U are first obtained by scanning an original. The sharp signal S represents density of each pixel. The unsharp signal U for each pixel represents average density of a certain area around each pixel. A basic enhancement signal (S-U) is obtained by subtracting the unsharp signal U from the sharp signal S. Usually, an enhancement signal k(S-U) is obtained by multiplying a coefficient k and the basic enhancement signal (S-U) to adjust the magnitude of the detail enhancement. An enhanced image signal of the original is finally produced by adding the enhancement signal k(S-U) and the sharp signal S, whereby sharpness of a reproduced image improves.
A process scanner often utilizes a method of directly reading a greater area than one pixel at a time to obtain the unsharp signal U. FIGS. 1A and 1B are diagrams showing weighting factors employed in the detail enhancement in a process scanner. FIG. 1A illustrates a weighting factor W.sub.1 which is constant over an averaging area R around a center pixel P. FIG. 1B illustrates another weighting factor W.sub.2 suggested by Japanese Patent Publication Gazette No. 39-24581. The weighting factor W.sub.2 is largest at the center pixel P and becomes smaller toward the periphery of the averaging area R. The process scanner produces the unsharp signal by optically obtaining weighted mean density over the averaging area R around each pixel P with the weighting factor W.sub.1 or W.sub.2.
In addition, Japanese Patent Laying Open Gazette No. 59 -141871 discloses a method of obtaining the unsharp signal by digital operation. FIG. 1C is a diagram showing a weighting factor W.sub.2 employed in this method. This method, while utilizing a buffer memory for storing an image signal with respect to several scanning lines, executes digital operation for obtaining the weighted means density over the averaging area R with the weighting factor W.sub.3, thereby obtaining the unsharp signal.
FIGS. 2A and 2B are graphs showing MTF (Modulation Transfer Function) characteristics of the sharp signal S, the unsharp signal U, the basic enhancement signal (S-U) and the enhanced image signal ES(=S+K(S-U)). The graphs of FIGS. 2A and 2B are obtained by using the weighting factors W.sub.1 and W.sub.2, respectively. Because the basic enhancement signal (S-U) has a similar contrast ratio CT to that of the sharp signal S at comparatively higher spatial frequency F.sub.S and the enhanced image signal ES is based on the sharp signal S and the basic enhancement signal (S-U), the enhanced image signal ES also has a considerable contrast ratio CT up to as high spatial frequency as the sharp signal S has.
The unsharp signal U.sub.1 of FIG. 2A is obtained by using a constant weighting factor, such as that of FIG. 1A. The upper limit frequencies f.sub.S, f.sub.U1 of the sharp signal S and the unsharp signal U.sub.1 depend on radii of apertures employed in reading an original to obtain the sharp signal S and the unsharp signal U.sub.1, respectively. A spatial frequency f.sub.l1 at which the contrast ratio CT starts to decrease depends on the shape of the weighting factor, and the like.
The unsharp signal U.sub.2 of FIG. 2B is obtained by using the weighting factor W.sub.2. The upper limit frequencies f.sub.U2 and f.sub.S depend on radii of apertures, similarly.
As can be seen in FIGS. 2A and 2B, the greater the coefficient k becomes, the greater the contrast ratio CT becomes at the spatial frequency under the upper limit frequency f.sub.S. This means that the detail enhancement is effective up to the upper limit frequency f.sub.S of the sharp signal S. However, in this case, roughness and granular noise in an original are also enhanced; this lowers quality of a reproduced image. Consequently, the coefficient k could not be set so great.
In order to produce an image to be seen sharp with the naked eye, the image signal is desirably enhanced at the middle range of the spatial frequency far below the upper limit frequency of the sharp signal S. According to the conventional methods, however, the image signal is enhanced at the higher range as well as the middle range of the spatial frequency. Therefore, noises included in an original image are also enhanced.